Theoretical Perspective: Psychometrics

Introduction

Human behaviour is enormously variable, depending on the inner state of a person, the demands and opportunities of a situation, and in dependence on a person's individual style of living, and on her/his interests, abilities, values, and motives. ‘Psychological assessment’ denotes the universe of systematic, scientifically based methods for describing, recording or interpreting a person's behaviour – provided that a method meets defined psychometric criteria, especially of reliability and validity. In this contribution basic concepts and psychometric methods employed in developing, evaluating and applying assessment methods are introduced.

Mental Measurement

The variations in behaviour and experience, which are captured in an assessment method, differ in the kind and source of psychological data accessed. These may include biographical information (like school records or employment history information), behaviour traces (products of a person's behaviour, like drawings), direct behaviour observation, behaviour ratings, segments of expressive behaviour (like handwriting or style of emotional expression in a person's face), so-called projective techniques (highly unstructured visual or other stimulus material, which a person is invited to interpret), interview data, questionnaire responses, so-called objective tests, or psychophysiological data. In this sense, psychological assessment encompasses a wide spectrum of methods, with ‘tests’ being but one type of method. As a technical term, ‘(objective) test’ refers to a sample of items, questions, problems or the like chosen to sample representatively a presumed universe of items, questions or problems indicative of the trait or state to be assessed (Pawlik, 2000: 382). The literature referenced in the section ‘Psychometric Assessment Theories’, introduces these assessment sources in all the necessary detail.

While the choice of an assessment methodology will depend on the nature of the assessment problem under study, the heuristic goals of assessment can be described independent of the method chosen. This goal can either be descriptive, prognostic, explanatory, or decision-oriented. Purely descriptive assessment is a relatively infrequent exception, where the assessment is conducted with the sole purpose of describing the current behavioural state of a person. Much more [Page 1020]frequent are assessments subserving a prognostic or explanatory goal. In the first case the assessment is conducted to predict the person's behaviour, state of feeling etc., at a future occasion or in another setting. Examples are psychological tests administered to predict a person's success in a training programme or under psychotherapy. Extending assessment results in the opposite direction of time is called explanatory: to infer from a person's assessment results her/his psychological state at an earlier stage of development or under past situational conditions. For most practical purposes, psychological assessment is primarily or additionally – decision-oriented. That is to say that the assessment results are employed to facilitate or improve decisions among alternative courses of action or treatment. These may refer to different types of educational or professional training, to different methods of psychological therapy, to alternative vocational choices, or the like.

Irrespective of the heuristic goal, assessment methods have to meet stringent criteria of accuracy (reliability) and validity. To this end, formal (i.e. mathematical-statistical) theories of measurement have been developed for the construction and the quality control of assessment methods. ‘Psycho-metrics’ is the branch of psychological methodology that comprises these theories and resulting methods for developing and evaluating assessment methods. In this context ‘measurement’ simply refers to a method of mapping variations in behaviour or experience onto a chosen number system. Different levels of measurement are distinguished depending on the rules and prerequisites chosen for this mapping.

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